The data points on this curve were obscured by the trend line. Changing the style of the marker makes the data points easier to see.
On a chart it's frequently difficult to see your data points if there's a trend line or other line obscuring them. To make your data points easier to see, you can change the marker style and size.
Though you can't see it in the default view (the background and foreground have the same color), some markers have a pattern on them. By changing the colors of the foreground or background you can see their patterns.
Changing the shape or the size of the markers can also help to make them stand out.
You'll also see how to do this in the practice session.
Note If you are publishing your chart in a scientific publication, you should only use black and white for the markers.